The present disclosure relates to a method of fabricating patterned metallic deposits on substrates and the articles so fabricated.
Polymeric films with patterns of metallic material have a wide variety of commercial applications. In some instances, it is desired that a conductive grid be sufficiently fine to be invisible to the unaided eye and supported on a transparent polymeric substrate. Transparent conductive sheets have a variety of uses including, for example, resistively heated windows, electromagnetic interference (EMI) shielding layers, static dissipating components, antennas, touch screens for computer displays, and surface electrodes for electrochromic windows, photovoltaic devices, electroluminescent devices, and liquid crystal displays.
The use of essentially transparent electrically conductive grids for such applications as EMI shielding is known. The grid can be formed from a network or screen of metal wires that are sandwiched or laminated between transparent sheets or embedded in substrates (U.S. Pat. Nos. 3,952,152; 4,179,797; 4,321,296; 4,381,421; 4,412,255). One disadvantage of using wire screens is the difficulty in handling very fine wires or in making and handling very fine wire screens. For example, a 20 micron diameter copper wire has a tensile strength of only one ounce (28 grams force) and is therefore easily damaged. Wire screens fabricated with wires of 20 micron diameter are available but are very expensive due to the difficulty in handling very fine wire.
Rather than embed a preexisting wire screen into a substrate, a conductive pattern can be fabricated in-situ by first forming a pattern of grooves or channels in a substrate and then filling the channels with a conductive material. This method has been used for making conductive circuit lines and patterns by a variety of means, although usually for lines and patterns on a relatively coarse scale. The grooves can be formed in the substrate by molding, embossing, or by lithographic techniques and then filling the grooves with conductive inks or epoxies (U.S. Pat. No. 5,462,624), with evaporated, sputtered, or plated metal (U.S. Pat. Nos. 3,891,514; 4,510,347; 5,595,943), with molten metal (U.S. Pat. No. 4,748,130), or with metal powder (U.S. Pat. Nos. 2,963,748; 3,075,280; 3,800,020; 4,614,837; 5,061,438; 5,094,811). Conductive grids on polymer films have been made by printing conductive pastes (U.S. Pat. No. 5,399,879) or by photolithography and etching (U.S. Pat. No. 6,433,481). These prior methods have limitations. For example, one problem with conductive inks or epoxies is that the electrical conductivity is dependent on the formation of contacts between adjacent conductive particles, and the overall conductivity is usually much less than that of solid metal. Vapor deposition of metal requires expensive equipment and electroplating presents challenges to uniformity and both often require a subsequent step to remove excess metal that is deposited between the grooves. Molten metal can be placed in the grooves but usually requires the deposition of some material in the grooves that the metal will wet. Otherwise the molten metal will not penetrate nor stay in the grooves due to surface tension of the molten metal.
Circuits have been made by placing metal powder into grooves followed by compacting the powder to enhance electrical contact between the particles. Lillie et al. (U.S. Pat. No. 5,061,438) and Kane et al. (U.S. Pat. No. 5,094,811) have used this method to form printed circuit boards. However, these methods are not practical for making fine circuits and metal patterns. On a fine scale, replacing or re-registering the tool over the embossed pattern to perform the metal compaction is difficult. For example, a sheet with a pattern of 20 micron wide channels would require that the tool be placed over the pattern to an accuracy of roughly three micrometers from one side of the sheet to the other. For many applications, the sheet may be on the order of 30 cm by 30 cm. Dimensional changes due to thermal contraction of a thermoplastic sheet are typically about one percent or more during cooling from the forming temperature to room temperature. Thus, for a 30 cm by 30 cm sheet, a contraction of one percent would give an overall shrinkage of 0.3 cm. This value is 1000 times larger than the three micrometer placement accuracy needed, making accurate repositioning of the tool difficult.